Printed circuit board

ABSTRACT

A method for producing a circuit board involves printing a U.V. curable ink onto a substrate in a desired circuit pattern and curing the ink by exposing it to a pulsed U.V. source or subjecting the circuit pattern prepared from a U.V. curable ink containing magnetite particles to a magnetic field to move the magnetite particles to the upper surface of the U.V. curable ink. Other embodiments include circuit boards made in accordance with these methods and the use of the U.V. curable ink as a shielding composition for enclosures housing electronic equipment.

This is a division of Application No. 07/365,712, filed June 14, 1989now U.S. Pat. No. 4,960,614, which is a CIP of Ser. No. 07/011,975,filed Feb. 6, 1987 now U.S. Pat. No. 4,863,757.

BACKGROUND OF THE INVENTION

In the past few years there has been a tremendous amount of activity inthe area of replacing "subtractive" printed circuit boards with"additive" printed circuit boards. There are major environmental,economic, and marketing reasons for the interest expressed in thistechnology. Two of the more important reasons are the growth of theelectronic industry and the environmental problems associated withtraditional copper "subtractive" circuit boards which consume resources,such as the copper foil itself, as well as the process itself whichgenerates hazardous waste.

In an effort to overcome or minimize the disadvantages associated withthe production of "subtractive" printed circuit boards, membrane switchcircuit boards were developed in the late seventies and early eighties.These circuit boards were generally silver loaded resin inks printed onpolyester films. However, they exhibited rather low voltage and currentcarrying capabilities and were employed principally in the field ofsimple switches and not as true printed circuit boards.

It is also important to distinguish additive technologies, such as CC-4boards which rely on electrodeless copper plating to achieve an additivecircuit board from the present invention. The former, which indeed is anadditive process, nonetheless continues to produce undesirable effluentseven though it offers some cost advantages over the more conventionalsubtractive techniques. On the other hand, the present invention notonly avoids the production of undesirable effluents but also provideseconomic advantages over systems heretofore employed in the productionof printed circuit boards.

Two of the more significant advantages secured by the present invention,not achievable heretofore, are (1) a low cost silver based ink providinglow resistance values and being U.V.-curable and (2) a low cost silverbased solderable ink which also provides low resistance and isU.V.-curable. In both instances the present invention provides a circuittrace whose cross section involves a U.V.-curable material incombination with either silver coated glass or silver coated magnetitespheres.

An inherent problem associated with U.V. technology resides in the factthat the U.V. material itself is non-conductive and represents asignificant percentage of the conductive ink composition. In many casesthis can be as high as 30 percent U.V.-curable resin and 70 percentconductive material.

While the advantages of the present invention are applicable to planarboards or substrates, it will be appreciated that these same advantagescan be secured with non-planar substrates such as, for instance computerkeyboards and the like.

The present invention thus relates to systems for securing the abovenoted advantages and for avoiding the disadvantages associated withknown methods of producing printed circuit boards.

One of these systems involves curing the U.V. curable resin component ofthe U.V. curable ink containing spherical conductive particles bysubjecting the same to a U.V. source in a pulsing manner.

Another of these systems involves the use of a magnetic field and whileunder the influence of the magnetic field curing the U.V. curable resincontaining spherical magnetite conductive particles by subjecting thesame to a U.V. source whether or not in a pulsing manner.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will be moreclearly appreciated from the following description taken in conjunctionwith the accompanying drawings in which:

FIG. 1 is a schematic cross-section through an ink film containingconductive spheres, which ink film is not treated in accordance with thepresent invention;

FIG. 2 is a schematic cross-section through an ink film also containingconductive spheres, which ink film has, in accordance with the presentinvention been subjected to a U.V. source in a pulsing manner or to aU.V. source under the influence of a magnetic field whether or not theU.V. source is applied in a pulsing manner;

FIG. 3 is a schematic view illustrating the packing of identical sphereswhereby adjacent layers thereof are capable of slipping past oneanother;

FIG. 4 is a schematic view illustrating the packing of identical sphereswhereby adjacent layers thereof are not capable of slipping past oneanother;

FIG. 5 is a schematic view illustrating the present invention whereinspheres vary in diameter by at least plus or minus 15 microns therebypermitting additional packing without undue reduction in fluidity yetproviding high conductivity;

FIG. 6 is a schematic of one embodiment of the magnetic device of thepresent invention;

FIG. 7 is a frontal schematic of another embodiment of a magnetic deviceof the present invention;

FIG. 8 is side view of the magnetic device of FIG. 7.

In FIG. 1 which illustrates a typical dispersion of spherical particles12 in a resin 10 the problem of having large interstices between theparticles filled with resin which is an insulator can be easily seen. Onthe other hand, in FIG. 2, which is representative of the presentinvention, the spherical particles 12 are closely packed with only asmall amount of resin 10 filling the interstices therebetween.

GENERAL DESCRIPTION OF A FIRST SYSTEM

The inventor has now discovered that when a U.V. source with an outputin the region between 360 nm and 420 nm is employed in a pulsed mode, ashrinkage of the conductive ink circuitry film occurs whereby shrinkageis facilitated evenly throughout the conductive ink film thickness. Thiscauses the conductive particles to move into closer contact with regardto one another, thus resulting in the conductive ink circuitry or tracebeing capable of carrying a greater operating current, as well aslowering the current resistance.

SPECIFIC DESCRIPTION OF THE FIRST SYSTEM

One embodiment of the present invention thus relates to a method forproducing a circuit board having conductive circuit elements with aspecific resistance of less than 0.05 ohm/cm² patterned on anon-conductive substrate comprising (a) printing a U.V. curable ink ontothe nonconductive substrate in a desired circuit pattern and (b)effecting a U.V. radiation cure of the U.V. curable ink by exposing saidU.V. curable ink to a U.V. source having an output in the region between360 nm and 420 nm. The exposure of the ink to the U.V. source iseffected in a pulsing manner which comprises 5 to 8 one-half secondexposure periods, each exposure period being immediately followed by anon-exposure period of about 2 to 3 seconds. When the ink is cured inaccordance with the present invention a shrinkage of the ink filmthickness occurs and is facilitated evenly throughout the conductive inkfilm thickness. This causes the particles to move into closer contactwith one another, thus resulting in the patterned conductive circuitelements being capable of carrying a greater operating current as wellas exhibiting a lower resistance.

The U.V. source employed in the present invention is electrodeless.Instead of using electrodes to feed energy into the discharge, thedischarge tube absorbs microwave energy via waveguides into a microwavechamber in which the tube is housed. The lamp system employed in thepresent invention is modular and consists of two parts, an irradiatorand a power supply. The irradiator contains a microwave chamber formedby an anodized aluminum reflector of semi-elliptical cross section withflat ends. The lamp itself is a closed, 10 inch-long tube of transparentvitreous silica varying in internal diameter from 8 mm near the ends to6 mm at the center. The lamp is located so that its axis lies at thefocus of the ellipse and it acts as a dissipative load. Microwave energyis generated by two 1500 watt magnetrons and is fed through waveguidesinto the chamber via rectangular slots cut in the back of the reflector.The microwave frequency used is 2450 MHz. The magnetrons and waveguidesare cooled by a filtered air flow and this air is also passed throughsmall circular holes cut in the back of the reflector, and over thelamp. In order to better disperse the output of the lamp and fully curethe conductive circuit elements, the surface of the reflector isprovided with 1 inch facets much like the surface of a golf ball.

An important factor which dictated the election of an electrodeless U.V.output source is the lack of lamp deterioration associated withelectrodes. This deterioration has prevented U.V. resins from being usedin a truly viable production of circuit boards of the type produced inaccordance with the present invention. One reason for this is that in anelectrode lamp the output wavelength will vary with time due toelectrode deterioration, and there is a direct relationship betweendegree of cure of the curable U.V. resin and the current carryingcapability of the conductive circuit elements produced therefrom. Theinventor has found a particularly effective electrodeless lamp is onewith iron iodide as a dopant which enhances the spectral output in thewavelength of 360 nm and 420 nm.

Suitable substrates on which the U.V. curable ink of the presentinvention can be printed, especially for use in membrane switches aregenerally organic polymer films having the properties of highflexibility, tensile strength, elasticity, dimensional stability andchemical inertness. Transparency is also a frequently desired propertyfor such materials. Materials meeting these criteria include polyolefinssuch as polyethylene and polypropylene, polycarbonates, polyesters andpolyvinyl halides such as polyvinyl chloride and polyvinyl fluoride. Themost highly preferred and most widely used substrate material formembrane switches is a polyester film, e.g. Mylar polyester film.

The U.V. curable ink employed in the present invention comprises fromabout 33 to 38 weight percent of a thermosetting resin binder and 67 to62 weight percent of spherical or spheroidal conductive particles havinga particle size distribution ranging from 1 to 30 microns.

Representative thermosetting resins usefully employed in the presentinvention include (1) phenolic resins such as those produced by reactingphenols with aldehydes; (2) amino resins such as the condensationproducts of urea and of melamine with formaldehyde; (3) unsaturatedpolyester resins wherein the dibasic acid or the glycol, or both,contain double bonded carbon atoms as exemplified by the following:##STR1##

Unsaturated acids include maleic anhydride or fumaric acid while whenthe unsaturation is supplied by the glycol, a saturated acid oranhydride such as phthalic anhydride or adipic, azelaic or isophthalicacid can be employed. Ethylene and propylene glycols are often employedbut 1,3- and 2,3-butylene, diethylene and dipropylene glycols are alsooften used. While styrene is commonly employed, other monomers usedinclude vinyl toluene, methyl methacrylate, diallyl phthalate andtriallyl cyanurate; (4) epoxy resins such as the condensation product ofepichlorohydrin with bisphenol A, although other hydroxyl-containingcompounds such as resorcinol, hydroquinone, glycols and glycerol can beemployed and (5) silicone polymers produced by intermolecularcondensation of silanols. Other thermosetting resins such as alkydresins including those based on phthalic anhydride and glycerol, orthose based on other polyhydric alcohols such as glycols,pentaerythritol or sorbitol, and other acids such as maleic anhydride,isophthalic and terephthalic acid can also be used. Still otherthermosetting resin binders include allyl resins, e.g. diallyl phthalateand allyl diglycol carbonate, as well as furane resins such as thosebased on furfuraldehyde in combination with phenol.

Preferably, the thermosetting resin binder employed in the presentinvention is a formulation of liquid acrylic modified monomers,oligomers and polymers activated by a combination of a ketonephotoinitiator and an amine. The resin is synthesized with either aterminal or pendant acrylate group, with a urethane being the preferredoligomer, as follows: ##STR2##

.sup.× --HD--AD]_(n) Polyester based on adiple acid (AD)and hexanediol(HD). Other suitable resins include acrylated epoxyresins, such asNovacure 3700, available from Interez, Inc., having the followingchemical formula ##STR3## acrylated polyethers, such as the followingpolyether based on 1, 2,6-hexane triol and propylene oxide ##STR4##acrylated polyesters, such as those formed from the esterification ofpolyhydric esters with acrylic acid to yield ##STR5## which specificallyis a polyester based upon adipic acid and hexanediol; and thio/ene andthio/acrylate systems, such as the polythiols developed by W. Grace andCompany, Ltds., ##STR6##

In order to achieve longer wavelength absorption in the range of 360 nmto 420 nm, a ketone amine adjuvant is employed. Preferably this adjuvantis Michler's ketone since it contains both ketone and aminefunctionality in one molecule. However, a mixture of benzophenone andMichler's ketone has been found to be particularly effective (1) wherethe two components are admixed prior to incorporation into the curableink vehicle, (2) where the spherical or spheroidal conductive particlesare silver coated glass spheres and (3) where the latter are present inthe U.V. curable ink in an amount greater than 60 weight percent basedon the total weight of the curable ink.

The spherical or spheroidal conductive particles employed in the presentinvention are preferably, silver coated glass spheres having thefollowing characteristics: average particle diameter -15 microns;average particle size distribution-1 to 30 microns; silver coating-12percent by weight based on the total weight of the spheres; particledensity-2.7 g/cc; specific surface area-0.178 m² /g; and minimum percentrounds by microscope-90.

Other particulated materials such as iron, zinc, nickel, copper and thelike can also be employed, these particulated materials having aparticle size distribution and an average particle size previouslydefined.

The output spectra of six electrodeless lamps were tested and analyzedfor their effectiveness in curing a U.V. curable ink in accordance withthe present invention. The results are given below.

    __________________________________________________________________________    Lamp A         Lamp D    Lamp M    Lamp M'   Lamp V   Lamp X                  Interval                                                                           Power                                                                             Power Power                                                                             Power Power                                                                             Power Power                                                                             Power Power                                                                             Power                                                                              Power                                                                             Power               (NM) (watts)                                                                           (accum)                                                                             (watts)                                                                           (accum)                                                                             (watts)                                                                           (accum)                                                                             (watts)                                                                           (accum)                                                                             (watts)                                                                           (accum)                                                                            (watts)                                                                           (accum)             __________________________________________________________________________    200-210                                                                             7.9                                                                               8     7.7                                                                               8    13.4                                                                               13    7.5                                                                               7     0.4                                                                               0    7.1                                                                               7                  210-220                                                                            17.0                                                                               26   15.2                                                                               23   42.4                                                                               56   20.9                                                                               28    1.4                                                                               2   22.1                                                                               29                 220-230                                                                            25.3                                                                               51   15.8                                                                               39   67.2                                                                              123   31.4                                                                               60    2.9                                                                               5   32.3                                                                               62                 230-240                                                                            23.6                                                                               75   14.6                                                                               53   46.2                                                                              170   27.9                                                                               88    3.6                                                                               8   41.3                                                                              103                 240-250                                                                            27.6                                                                              102   24.8                                                                               78   303 200   29.8                                                                              118    7.3                                                                               16  34.5                                                                              137                 250-260                                                                            55.7                                                                              158   43.1                                                                              121   101.2                                                                             301   73.1                                                                              191   11.5                                                                               27  55.7                                                                              193                 260-270                                                                            38.9                                                                              197   32.1                                                                              153   78.1                                                                              379   79.5                                                                              270   12.5                                                                               40  48.3                                                                              241                 270-280                                                                            48.8                                                                              246   42.9                                                                              196   34.7                                                                              414   31.7                                                                              302   12.3                                                                               52  29.3                                                                              271                 280-290                                                                            91.0                                                                              329   24.6                                                                              221   28.7                                                                              443   85.1                                                                              387   26.2                                                                               78  38.2                                                                              309                 290-300                                                                            39.3                                                                              366   48.6                                                                              269   43.5                                                                              486   26.8                                                                              414   46.5                                                                              125  38.8                                                                              348                 300-310                                                                            72.0                                                                               73   56.7                                                                               57   46.2                                                                               46   20.9                                                                               21   16.0                                                                               16  29.1                                                                               29                 310-320                                                                            77.6                                                                              150   44.2                                                                              101   92.1                                                                              138   42.5                                                                               63   17.3                                                                               33  51.4                                                                               80                 320-330                                                                            64.5                                                                              215   35.5                                                                              136    9.0                                                                              147   10.1                                                                               74   16.4                                                                               50  26.4                                                                              107                 330-340                                                                            25.6                                                                              240   20.3                                                                              156   18.4                                                                              166   10.8                                                                               84   20.2                                                                               70  25.9                                                                              133                 340-350                                                                             9.3                                                                              250   43.2                                                                              200    5.4                                                                              171    6.9                                                                               91   22.3                                                                               92  53.1                                                                              186                 350-360                                                                            48.4                                                                              298   78.0                                                                              279    5.2                                                                              176   26.0                                                                              117   24.4                                                                              117  25.2                                                                              211                 360-370                                                                            58.6                                                                              357   93.3                                                                              373   118.9                                                                             293   173.4                                                                             291   35.1                                                                              152  112.1                                                                             323                 370-380                                                                            25.2                                                                              382   115.2                                                                             488    8.0                                                                              307   40.2                                                                              331   29.3                                                                              181  15.5                                                                              339                 380-390                                                                            37.1                                                                              419   112.1                                                                             600    6.3                                                                              310    9.1                                                                              340   31.5                                                                              213  15.4                                                                              354                 390-400                                                                            11.5                                                                              430   41.2                                                                              641    5.9                                                                              315    8.5                                                                              348   35.4                                                                              240  15.4                                                                              370                 400-410                                                                            92.9                                                                               93   46.9                                                                               47   50.5                                                                               50   133.0                                                                             134   135.1                                                                             135  40.1                                                                               40                 410-420                                                                            10.1                                                                              103   33.5                                                                               80    7.0                                                                               57   17.1                                                                              151   144.0                                                                             279  15.3                                                                               55                 420-430                                                                            15.5                                                                              119   44.6                                                                              125    7.9                                                                               65    8.6                                                                              160   76.6                                                                              256  18.5                                                                               74                 430-440                                                                            41.0                                                                              160   61.7                                                                              187   79.5                                                                              145   42.9                                                                              203   44.9                                                                              401  71.0                                                                              145                 440-450                                                                            30.1                                                                              190   28.2                                                                              215    8.8                                                                              154   11.1                                                                              214   32.5                                                                              433  29.3                                                                              174                 __________________________________________________________________________

Curing mechanism of a U.V. curable acrylate resin utilized in theinvention.

    ______________________________________                                        Typical Formulation                                                                        CP - 100       Function                                          ______________________________________                                        Monomer      Acrylic monomers                                                                             Film-forming                                      Pre-polymer  oligomers, polymers                                                                          materials                                         Photo-initiator                                                                            mixed ketones  light sensitive                                                with amine     chemical                                          Surfactant   non-ionic type wetting agent                                     Additives    Silica         suspending agent                                                              gloss reduction                                   ______________________________________                                    

An unsaturated polyester mixed with its monomer can be cross linked byU.V. light if a suitable photoinitiator is incorporated. The doublebonds in the unsaturated esters provide potential bonding sites forpolymerization by free radical processes. Acrylate esters polymerize ata rate which is at least an order of magnitude greater than is foundwith other unsaturated esters. The pre-polymers are normally veryviscous, or even solid, and in order to reduce the viscosity it has benfound convenient to use a diluent. Monoacrylates and some oligomers,with low volatility have been employed. Suitable monomeric diluentsinclude ##STR7##

2. acrylics, including

(i) Monoacrylates

e.g. n-Butyl acrylate

2-Ethyl hexyl acrylate (EHA) ##STR8## phenoxyethyl acrylate Tetrahydrylfurfuryl acrylate

2-Hydroxy ethyl acrylate ##STR9## cyclohexyl acrylate3-Butoxy-2-hydroxypropyl acrylate

(ii) diacylates ##STR10##

(iii) Triacrylates ##STR11##

(iv) Tetra-acrylates ##STR12##

(v) Penta-acrylates ##STR13##

Benzophenone and its diaryl ketone derivatives possess the unifyingfeature of producing initiator radicals by intermolecular abstractionfrom an H-donor after irradiation with a U.V. light source. TheH-abstraction step produces two radicals both of which are potentialinitiators of radical initiated polymerization. Tertiary amines witha-H-atoms react readily with the excited states of the ketones.H-transfer may be preceded by rapid formation of an excited statecomplex (exciplex) between the amine and excited ketone. Michler'sketone possesses both a diaryl ketone group and tertiary amine group.Combinations of Michler's ketone and benzophenone have been reported toexhibit a synergism when utilized in a U.V. curing of printing inks.

This synergism is believed to arise from higher absorptivity ofMichler's ketone together with the greater reactivity of excitedbenzophenone.

In addition to the formation of an exciplex to enhance thephotopolymerization rate, amines, such as small amounts oftriethylamine, have other advantages in a benzophenone/acrylate system.

The α-amino (R₂ C-NR₂) radical, formed after the H-abstraction step, isgenerally much more effective than the relatively stable and bulkyketone radical. Besides, α-amino radicals are electron-rich due to theresonance effect of the adjacent heteroatom and initiation is consideredto be much more efficient with the electron-poor monomers, such asacrylates.

The addition of oxygen to growing polymers will form relatively lessreactive peroxy radicals which will cause the radical-radical reactions,terminate the polymerization processes, and result in short chainlengths. This factor as well as oxygen quenching of triplet ketones islargely responsible for air inhibitors of surface-cure. However, thesedeleterious effects of oxygen are minimized by amine co-initiation sincethe c-amino radicals can consume oxygen by a chain process such as:##STR14##

These features together make the combination of ketone/tertiary amine aparticularly effective photoinitiator system for U.V. curing in air.

Mechanisms

The following mechanisms are involved in the preceding reactions##STR15##

As noted above, when an acrylate system, such as CP-100 is employed itis cured by photo initiated free radical polymerization. Thephotoinitiator is usually an aromatic ketone with the concentrationabout 4-5%. Suitable photoinitiators include: ##STR16##

These are generally used in conjunction with one of the synergisticagents, such as:

ethyl-4-dimethylamino benzoate

ethyl-2-dimethyl amino benzoate

2-(n-butoxy) ethyl-4-dimethylamino benzoate

2-(dimethylamino)ethylbenzoate ##STR17##

(8) Organic Sulphur Compounds

e.g. diaryl disulphide

dibenzoyl disulphide

diacetyl disulphide

(9) Organic Phosphorus - containing compounds

e.g. Triphenyl phosphine

Triphenyl phosphite

Tri-orthotolyl phosphine

(10) Chlorosilanes

e.g. trimethyl chlorosilane

(11) Azo compounds

e.g. azo-bis (isobutyronitrile)

diazirine

The excited aromatic ketone after being irradiated with a U.V. sourcewill get an H-atom from a monomer, solvent, or preferably a tertiaryamine with an a-hydrogen. The H-transfer reactions between the aromaticketones and the amines are usually very fast. The resulting α-aminoradical can consume 0₂ molecules through a chain process and regeneratethe α-amino radicals. This process can assist surface curing, where thecuring film contacts the air. O₂ molecules are very effective quenchersfor the radicals.

A small amount of Michler's ketone is also mixed into the formulation(about 1/10 of the concentration of benzophenone). Michler's ketone hasboth the aromatic ketone group and tertiary amine group in one molecule,and has strong absorption of light around 350 nm. Accordingly, it caneffectively absorb the U.V. light from Hg-lamp and pass the energy tobenzophenone to form excited benzophenone.

In the curing of a CP-100 system which is generally employed because thecure speed of the double bond in the acrylate group, the free radicalgenerated from the photoinitiator will react with the unsaturated doublebond in the polymer chain and then the other free radical is formed,which will react with the second unsaturated polymer chain to form thecrosslinked thermosetting polymers.

Benzophenone exhibits absorption maxima in the ultraviolet spectraregion at about 250 and 350 nm with e values of approximately 15,000 and100 respectively. The e values represent a measure of the probability oflight absorption at each wavelength. With benzophenone present in thisfirst system, most of the 250 nm is absorbed at or near the surface,whereas the 350 nm light is available throughout the film for thethrough-cure. Michler's ketone, however, exhibits e values of about15,000 and 40,000 at 250 nm and 350 nm respectively. The combination ofMichler's ketone and benzophenone shows some kind of synergism, probablybecause of the higher absorptivity of Michler's ketone and the greaterreactivity of triplet benzophenone.

The free radicals generated from the photoinitiation step or thepropagation process are very reactive. They will be quenched effectivelyby O₂ molecules, recombine with other radicals nearby, or undergo O₂addition and terminate the propagation. In general, as soon as exposureto the U.V. source is terminated the polymerization processes stop.There is an optimal concentration of photoinitiator which is governed byefficient U.V. light utilization and initiator radical formation asopposed to self-quenching and light U.V. screening by thephotoinitiator.

Most acrylic functional resins are extremely viscous due to the urethaneor epoxy backbones. Among these it has been found that epoxy resin hasgood adhesion, a high level of chemical resistance, non-yellowing colorsand flexibility. Polyesters and polyethers have lower viscosities. Thepolymerizable resins can provide the final film hardness and chemicalresistance. The reactive monomers, or the unreactive plasticizers, areoften introduced to modify its flow properties and reduce the final filmbrittleness. Reactive monomers can be used not just as rheological(viscosity and tack) control agents but also as crosslinking agents.

A peculiar effect has been discovered which is significant in the actualpacking of the spheres, whether the pulsing mode of this first system isemployed or whether the magnetic field of the second system is used toeffect ink film shrinkage.

The closest packing of identical spheres 12 with the resin 10 fillingthe interstices is a completely hexagonal array with all spheres incontact. This array however has no fluidity because adjacent layerscannot slide past one another. See FIG. 4. If, however, one considers aset of planar arrays of spheres 12, each of which has hexagonal packing,but now each sphere 12 rests in registry with the one below it ratherthan nesting in a space defined by three spheres 12 of the adjacentlayer, it is now possible for slippage to take place. See FIG. 3. If allthe spheres 12 are of equal diameter it is only possible for the spheres12 to occupy a volume fraction of slightly more than 60 percent whichyields insufficient electrical conductivity. However, if the spheres 12vary in diameter by at least plus or minus 15 microns, increased packingwithout undue reduction of fluidity is achieved. See FIG. 5.

General Description of the Second System

The inventor has also discovered that when a U.V. curable ink comprisinga suspension of silver-coated magnetite particles in a U.V. curableresin is employed and a circuit pattern printed with this U.V. curableink composition is subjected to a magnetic field of an intensitysufficient to move the magnetite particles to a position at or near theupper surface of the resin, i.e., the surface remote from thatjuxtaposed to the circuit board substrate on which the circuit patternis printed, without breaking the surface tension thereof orsubstantially increasing the thickness of the ink film, and effectingU.V. radiation cure of the U.V. curable ink, an ideal printed circuitboard is achieved.

This second system, as does the previously described first system,causes the magnetite particles to move into closer contact with oneanother, thus resulting in the patterned conductive circuit elements ortrace being capable of carrying a greater operating current as well asexhibiting a lower resistance and being solderable.

Polymer thick film and "additive" printed circuit board technologyappear destined to grow at a rapid rate. The impetus for this growth isdue to several reasons, amongst which are (a) the development of adirectly solderable conductor which is one of the primary benefitsachieved by the present invention, and especially the implementation ofthe inventor's second system, which eliminates the need of plating, and(b) the increasing use of surface-mount technology since the capabilityof fabricating structures using polymer thick technology and surfacemount technology make a very attractive combination in terms of size andcost when compared to multilayer printed circuit boards with a multitudeof plated through holes.

Any resin may be employed which is U.V. curable, including:cycloaliphatic epoxides which are commercially available as UVR-6100 andUVR-6110 from Union-Carbide Corporation and CY-179 from Ciba-GeigyCorporation, where UVR-6110 and CY-179 are3,4-epoxycyclohexylmethyl-3,4-epoxycylohexane carboxylate having thefollowing structural formula: ##STR18## Novolak epoxy resins, includingthose derived from ortho-cresol formaldehyde novolac and epichlorohydrin##STR19## which is commercially available as ECN-1235 from Ciba-GeigyCorporation and ##STR20## which is commercially available as D.E.N. 438from the Dow Chemical Company; diglycidyl ethers of bisphenol A (DGEBA)##STR21## commercially available as Araldite GY 6010 from Ciba-GeigyCorporation, D.E.R. 331 from The Dow Chemical Company, EPI-REZ 510 fromInterez, Inc. and EPON 828 from Shell Chemical Company; diacrylate esterof Bisphenol A type epoxy resins ##STR22## commercially available asNovacure 3700 from Interez, Inc.; partially acrylated bisphenol A typeepoxy resins, ##STR23## commercially available as RDX 52197from Interez,inc.; polyglycol diepoxides ##STR24## commercially available as D.E.R.736 from The Dow Chemical Company; diacrylated ester of a polyglycoltype epoxy resin ##STR25## partially acrylated ester of polyglycol typeepoxy resin ##STR26## diglycidyl ethers of phthalic acid esters##STR27## such as Syodyne-508 commercially available from Showa Denco;diacrylate ester of phthalic acid type epoxy resins ##STR28## andpartially acrylated phthalic acid type epoxy resins. ##STR29##

In one embodiment of this second system the U.V. curable inkcompositions utilize as the polymeric matrix or binder a cycloaliphaticepoxide that can be cured in seconds with photoinitiators to a harddurable condition.

Modifiers can be included in the composition to improve flexibility andadhesion. Suitable flexibilizers include epoxide flexibilizers,commercially available as Cyracure UVR 6351 and Cyracure UVR 6379 fromUnion Carbide Corporation, caprolactone-based multifunctional polyolswhich are available as TONE Polyols from Union Carbide Corporation and apolytetramethylene oxide glycol available commercially as Polymeg 2000from QO Chemicals.

It may be desirable to include flow control agents or surfactants,examples of which include polyalkylene oxide modifieddimethylpolysiloxanes ##STR30## commercially available s SILWET L-7604from Union Carbide Corporation and a fluorinated allyl alkoxylateavailable as Fluorad FC-430 and FC-171 from 3M Company.

Fillers and other additives may be added. For example, to increase theviscosity it may be desirable to use inert polymers of cellulosics suchas cellulose acetate butyrate and ethyl cellulose, polycaprolactone andvinyl chloride/vinyl acetate copolymers; or silicas, such as anhydrousaluminum silicates commercially available as Optiwhite from BurgessPigment Company and zirconium silicates commercially available asExcelopax from NL Industries, Inc. To improve hardness, crystallinequartz, such as Minusil 15) available from PPG Industries, Inc., may beadded.

Diluents may also be added and include Cyracure UVR-6200 commerciallyavailable from Union Carbide Corporation, glycidyl acrylate, ##STR31##and glycidyl methacrylate ##STR32## both available from Aldrich ChemicalCompany, 3, 4-epoxycyclohexymethyl acrylate ##STR33##3,4-epoxycyclohexylmethyl methacrylate ##STR34## aliphatic triglycidylether available as EPI-REZ-5048 from Interez, Inc. and Araldite RD-2from Ciba-Geigy Corporation.

It is important to note that again one significant feature of thissecond system is the ability to separate the polymeric binder from theconductive particles via magnetic levitation of the silver-coatedmagnetite particles. The cycloaliphatic epoxide can be cured in secondswith an appropriate photoinitiator and U.V. light source.

The photoinitiators dissociate under the influence of U.V. radiation toform cationic species that rapidly polymerizes the cycloaliphaticepoxides. Unlike U.V. resins that are based on free radical chainreactions, cationic homopolymerization has few, if any, terminatingreactions. The propagation ends remain intact to form a "living"polymer; thus polymerization continues after U.V. exposure (even undersurface mount technology conditions).

Suitable photoinitiators for the U.V. curing of the cycloaliphaticepoxide are the various onimum salts that undergo photodecomposition toyield a cationic species for initiation and propagation of thepolymerization. Photogenerated HPF6 is a strong protonic acid that caninitiate the cationic polymerization. Other suitable photoinitatorsinclude aryldiazonium compounds having the following formula ##STR35##where X⁻ =BF₄ ⁻, PF₆ ⁻, AsF₆, SbF₆ ⁻, FeCl₄ ⁻, or SbCl₆ ⁻ ;diaryliodonium compounds

(Ph₂ I)⁺ X⁻

where X⁻ =BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, or SbF₆ ⁻ ; triarylsulphonium compounds(Ph₃ S) ⁺ X⁻ commercially available as Cyracure UVI-6974 and CyracureUVI-6990 from Union Carbide Corporation, UVE-1014 and UVE-1016 fromGeneral Electric Co. and FX 512 from 3M Company; and triarylsilenoniumcompounds (PhSe) ⁺ X⁻, where in both triaryl compounds, X⁻ =BF₄ ⁻, PF⁻,A3F₆ ⁻ or SbF₆ ⁻.

Preferably, the cylcoaliphatic epoxides employed in the presentinvention are those which are commercially available such as UVE-1014sold by General Electric, FC-508 sold by 3M, and CP-101 manufactured byKey-Tech, with UVE-1014 being preferred. CP-101 is a multipurposecycloaliphatic epoxide monomer having excellent response to cure withphotoinitiators. As the major component CP-101 provides good adhesionand to avoid any brittleness that might be encountered with the use ofCP-101 it has been found advantageous to employ a high molecular weightpolyol plasticizer, such as Polymeg 2000. This particular plasticizeractually enters into the polymerization as shown by the followingreaction scheme: ##STR36##

It is believed that the curing mechanism involving a U.V. curable epoxyresin having a typical formulation below, is as follows

    ______________________________________                                        Typical Formulation  Function                                                 ______________________________________                                        Monomer      Cycloaliphatic                                                                            Film-forming materials                                            Epoxide                                                          Modifier     polyether   React with basic                                                  polyol      materials                                                                     Make coating flexible                                Photoinitiator                                                                             Cationic Type                                                                             Light sensitive                                                               chemical                                             Surfactant   Fluorinated Wetting agent for                                                 Chemical    non-porous substrates                                ______________________________________                                    

In the photoinitiation stage several inorganic and organometallic saltsare active photoinitiators of the cationic polymerization. Atriarylsulfonium compound almost approaches an ideal forphotoinitiators. This class of compounds possesses the favorableproperties of neither undergoing air inhibition, nor being temperaturesensitive or affected by other radical inhibitors. The photo-reactivityis not quenched by triplet-state quenchers and is not accelerated byradical photoinitiators. The photochemical mechanism is similar to thatof another class of compounds, i.e., diaryliodonium salts, buttriarylsulfonium salts have greater thermal stability. These salts havethe general structure

    Ar.sub.3 S.sup.+ MX.sub.n --

where MXn- is a complex metal halide, BF4--, PF6--, AsF6-- or SbF6--.The reactivity of salts is found to increase with the size of thecounter anion, namely, BF4-<<PF6-<AsF6<SbF6--.

Upon irradiation by the U.V. source having a wavelength below 350 nm,the sulfonium cation undergoes homolytic cleavage with the anionremaining unchanged. ##STR37## wherein Y-H represents a monomer orsolvent.

The overall photolysis reaction is

    Ar3S+MXn--+Y--H Ar2S+Ar2S+Ar.+Y+HMXn

A strong Bronsted acid for cationic curing such as HBF4, HPF6, or HSbF6is formed. The rate of photolysis of triphenylsulfonium salts is linearwith respect to the light intensity.

Using triphenylsulfonium salt photoinitiators, it has been foundpossible to polymerize virtually any cationically polymerizable monomer.This includes olefins, dienes, epoxides, cyclic ethers, sulfides,acetals, and lactones. Epoxy compounds and resins are of particularinterest as a class of polymerizable materials in U.V. curing. Ingeneral, these materials are readily available as commodity items, andthe resulting cured polymers possess excellent dimensional and thermalstability as well as superior mechanical strength and chemicalresistance.

Especially preferred epoxides for use in the present invention are thecycloaliphatic and diglycidyl ether of bisphenol A(DBEGA) types.Cycloaliphatic epoxides give faster cure response and are lower inviscosity, although they may not be as economical as DGEBA epoxides.However, both types of epoxides provide toughness, hardness and chemicalresistance.

Useful polyol plasticizers, which can contain either polyether orpolyester backbones, are usually mixed into the formulation to make thecoating more flexible. Polyester polyols give a faster cure response andare useful at higher levels to give excellent coating flexibility.Polyether polyols produce lower viscosity coatings and maintain greaterhydrolytic resistance to cured films. This latter type of polyol is nota typical plasticizer since it actually enters into the polymerization.

As indicated above, unlike U.V. coatings based on free radical chainreactions, cationic polymerization has few terminating reactions. Thepropagating ends remain intact to form a "living" polymer; thus,polymerization continues after U.V. exposure, i.e., under darkconditions. Immediately after irradiation at room temperature the resin,depending on light intensity, photoinitiator concentration, andtemperature, may show some tack or may not be fully solvent resistant.In general, the full chemical and physical properties of the resin doesnot develop for about 24 hours. This "post cure" can be markedlyaccelerated by raising the temperature. Similar properties of the curedfilms have been reached by warming the films at 71 C for 2-4 hours.

In the curing of an epoxide system, the mechanism can involve thefollowing specific type reaction scheme: ##STR38##

On exposure to actinic sources the photoinitiator of a U.V.-curingepoxide system will form a strong Bronsted acid or a Lewis acid, whichwill protonate the epoxide ring and make it readily accessible for anucleophilic attack. The nucleophile in this system is a hydroxyl groupfrom the polyol stabilizer or a monomer. This reaction will make ana-alkoxy and a free proton. The proton will protonate the other epoxidering to propagate the polymerization and the hydroxyl group (alcohol)will attack protonated rings to form crosslinked polymers. Thephotocationic systems have several advantages.

(1) They can be used to cure saturated monomers such as epoxy resins.The advantage of curing saturated epoxides over the unsaturated types isthat the former have only a small volatility, good flow, no significantcolor, negligible toxicity and superb physical and chemical properties;

(2) cationic photopolymerization is insensitive to aerobic conditions,and inert blanketing required for some free radical polymerizations isnot needed; and

(3) on removal of actinic radiation, these systems continue topolymerize thermally.

However, it has been found that post cure is usually required and thatbasic materials will neutralize the acids and should not be mixed intothe system.

In order to establish a useful magnetic source strength the movement ofone silver-coated magnetite sphere in resins of varying densities hasbeen calculated (see Tables IA-IVA). The material employed was theresult of printing sample circuit patterns or traces on a 0.005"polyester film anchored in a holding jig which was placed in proximityto the magnet plane. This holding jig or device was attached to amicrometer ball slide which allowed precise adjustability with respectto the position of the printed polyester sheet and the magnet. The dataset forth in Table III establishes the operational parameters forcorrect base to pole distance, time interval in magnetic field, resinviscosity, magnetic source strength and volume percentage of magnetitespheres in the resin.

                  TABLE IA                                                        ______________________________________                                        Rise time                                                                              mks   Symbol        cgs-equivalent                                   ______________________________________                                        Sphere                                                                        Diameter   m       2a     3.70E-03 0.0037 cm                                  Volume     m.sup.3 V      1.59E-13                                            Density    Kg/m.sup.3                                                                            ds     3.10E+03 3.10 g/cc                                  Mass       Kg      m      4.93E-10                                            Weight     N       sw     4.83E-09                                            Resin                                                                         Viscosity  Kg/sm   eta    5.00E+04 5000 c poise                               Thickness  m       R      1.78E-05 0.0007 in                                  Width      m       wd     1.27E-03 0.05 in                                    Density    Kg/m.sup.3                                                                            dr     2.50E+03 2.5 g/cc                                   Eff. of sphere                                                                           N       rw -   9.55E-11                                            Buoyant accel.                                                                           M/S.sup.2                                                                             W      1.09E+00                                            Substrate                                                                     Thickness  m       S      0.002                                               Magnetic Field                                                                Parameters                                                                    Source Strength                                                                          Vs/m.sup.2                                                                            Q      0.05     500 gauss                                  Base-to-pole                                                                             m       D      0.003    0.3 cm                                     distance                                                                      Permeability              1.69E+00                                            of spheres                                                                    ______________________________________                                    

                  TABLE IIA                                                       ______________________________________                                        Magnetization of Ferrite Sphere                                               B. Gauss   M emu   ! Avg.    B Vs/m.sup.2                                     ______________________________________                                         0         0                        0                                         100         4.9    1.616            0.0100                                    200        12.3    1.773            0.0200                                    350        20.4    1.732            0.0350                                    500        28.4    1.714            0.0500                                    650        35.7    1.690            0.0650                                    800        42.0    1.672            0.0800                                    950        49.2    1.651            0.0950                                    1044       55.0    1.662     1.689  0.1044                                    2045       76.3    1.469            0.2045                                    3062       81.2    1.333            0.3062                                    4067       82.7    1.256            0.4067                                    8079       84.3    1.131            0.0879                                    11943      84.5    1.089            1.1943                                    ______________________________________                                    

                  TABLE IIIA                                                      ______________________________________                                        Calculation of Rise (Fall) Time For                                           Magnetic-Field-Free Conditions                                                ______________________________________                                        Terminal Velocity in resin                                                                       2.77E-10 m/s m/s                                           t-zero             2.83E-11     s                                             Settling time      6.41E+04     s                                             ______________________________________                                    

                                      TABLE IVA                                   __________________________________________________________________________    CALCULATION OF THE RISE-TIME WITH AN APPLIED MAGNETIC FIELD                   __________________________________________________________________________    Delta-t. s                              Constant 3.75E-08                     time in   0.0005 ×                                                                        s velocity                                                                           B(x)    grad B Acceleration components in m/s 2      units of delta-t                                                                        m       m/s    Vs/sq.m Vs/cu.m                                                                              viscous magnetic                                                                             gravity                __________________________________________________________________________     0        2.000E-03                                                                             0.00E+00                                                                             2.60E-01                                                                              4.00E+00                                                                              0.00E+00                                                                             3.90E-08                                                                             4.83E-09                1        2.000E-03                                                                             2.19E-11                                                                             2.60E-01                                                                              4.00E+00                                                                             -3.82E-10                                                                             1.04E+00                                                                             4.83E-09                2        2.000E-03                                                                             5.20E-04                                                                             2.60E-01                                                                              4.00E+00                                                                             -9.07E-03                                                                             1.04E+00                                                                             4.83E-09                3        2.001E-03                                                                             1.04E-03                                                                             2.60E-01                                                                              4.00E+00                                                                             -1.81E-02                                                                             1.04E+00                                                                             4.83E-09                4        2.001E-03                                                                             1.55E-03                                                                             2.60E-01                                                                              4.01E+00                                                                             -2.70E-02                                                                             1.04E+00                                                                             4.83E-09                5        2.002E-03                                                                             2.06E-03                                                                             2.61E-01                                                                              4.01E+00                                                                             -3.58E-02                                                                             1.05E+00                                                                             4.83E-09                6        2.003E-03                                                                             2.56E-03                                                                             2.61E-01                                                                              4.02E+00                                                                             -4.46E-02                                                                             1.05E+00                                                                             4.83E-09                7        2.005E-03                                                                             3.06E-03                                                                             2.61E-01                                                                              4.02E+00                                                                             -5.34E-02                                                                             1.05E+00                                                                             4.83E-09                8        2.006E-03                                                                             3.56E-03                                                                             2.62E-01                                                                              4.03E+00                                                                             -6.21E-02                                                                             1.06E+00                                                                             4.83E-09                9        2.008E-03                                                                             4.06E-03                                                                             2.62E-01                                                                              4.04E+00                                                                             -7.07E-02                                                                             1.07E+00                                                                             4.83E-09               10        2.010E-03                                                                             4.55E-03                                                                             2.63E-01                                                                              4.05E+00                                                                             -7.94E-02                                                                             1.07E+00                                                                             4.83E-09               11        2.013E-03                                                                             5.05E-03                                                                             2.64E-01                                                                              4.07E+00                                                                             -8.80E-02                                                                             1.08E+00                                                                             4.83E-09               12        2.015E-03                                                                             5.54E-03                                                                             2.65E-01                                                                              4.08E+00                                                                             -9.66E-02                                                                             1.09E+00                                                                             4.83E-09               13        2.018E-03                                                                             NA     2.65E-01                                                                              4.10E+00                                                                             NA      NA     4.83E-09               14        NA      NA     NA      NA     NA      NA     4.83E-09               15        NA      NA     NA      NA     NA      NA     4.83E-09               16        NA      NA     NA      NA     NA      NA     4.83E-09               17        NA      NA     NA      NA     NA      NA     4.83E-09               18        NA      NA     NA      NA     NA      NA     4.83E-09               19        NA      NA     NA      NA     NA      NA     4.83E-09               20        NA      NA     NA      NA     NA      NA     4.83E-09               21        NA      NA     NA      NA     NA      NA     4.83E-09               22        NA      NA     NA      NA     NA      NA     4.83E-09               23        NA      NA     NA      NA     NA      NA     4.83E-09               24        NA      NA     NA      NA     NA      NA     4.83E-09               25        NA      NA     NA      NA     NA      NA     4.83E-09               26        NA      NA     NA      NA     NA      NA     4.83E-09               27        NA      NA     NA      NA     NA      NA     4.83E-09               28        NA      NA     NA      NA     NA      NA     4.83E-09               29        NA      NA     NA      NA     NA      NA     4.83E-09               __________________________________________________________________________                               Delta-t. s time                                                                          DELTA-V delta-x  total delta-x                                     in units of delta-t                                                                      m/s     m        m                      __________________________________________________________________________                                0         2.19E-11                                                                              1.10E-14 1.10E-14                                           1         5.20E-04                                                                              1.30E-07 1.30E-07                                           2         5.16E.04                                                                              3.89E-07 5.19E-07                                           3         5.12E-04                                                                              6.46E-07 1.16E-06                                           4         5.08E-04                                                                              9.01E-07 2.07E-06                                           5         5.05E-04                                                                              1.15E-06 3.22E-06                                           6         5.02E-04                                                                              1.41E-06 4.62E-06                                           7         4.99E-09                                                                              1.66E-06 6.28E-06                                           8         4.97E-04                                                                              1.90E-06 8.18E-06                                           9         4.95E-04                                                                              2.15E-06 1.03E-05                                          10         4.93E-04                                                                              2.40E-06 1.27E-05                                          11         4.92E-04                                                                              2.65E-06 1.54E-05                                          12         4.91E-04                                                                              2.89E-06 1.83E-05                                          13         NA      NA       NA                                                14         NA      NA       NA                                                15         NA      NA       NA                                                16         NA      NA       NA                                                17         NA      NA       NA                                                18         NA      NA       NA                                                19         NA      NA       NA                                                20         NA      NA       NA                                                21         NA      NA       NA                                                22         NA      NA       NA                                                23         NA      NA       NA                                                24         NA      NA       NA                                                25         NA      NA       NA                                                26         NA      NA       NA                                                27         NA      NA       NA                                                28         NA      NA       NA                                                29         NA      NA       NA                     __________________________________________________________________________

Depending upon the conductivity levels required the U.V. curable inkcomposition comprises 25 to 67 volume percentage of silver-coatedmagnetite spheres to the resin employed. This volume percentage can varydepending upon the electrical characteristics required in the circuitdesign. Thus the U.V. curable ink composition provides great flexibilityof loading which lends it to many printing or application techniquesincluding screen printing, gravure printing, spraying or nozzledistribution.

In some situations it has been found desirable to combine free radicaland cationic polymerization in the same U.V. curable ink compositions.Sulfonium salt is capable of initiating both free radical and cationicpolymerization, and therefore, simultaneous polymerization of acrylatesand epoxides.

As shown in FIG. 6 immediately after printing the circuit trace on thesubstrate, the resulting circuit traces 14 on the circuit board areplaced trace side down to the face 16 of the magnet 18. The preferreddistance from the highest point on the printed circuit trace to themagnet face is 0.008 inch and this is measured from the center 20 of themagnet face 16 This is particularly important when the magnet employedhas a spherically formed face so as to insure correct field strengthacross the circuit trace. Dwell time, or exposure of the circuit traceto the magnetic field can vary depending on the viscosity of the resinmatrix or binder. However, it has been found that using a resin having aviscosity of 20,000 centipoises, a dwell or exposure period of 3 secondsis sufficient to compact the magnetite spheres at the upper levelthereof in such a way to increase the conductivity thereof to thedesired levels.

The next step is to expose the circuit board to a U.V. source of 380 nmfor a period of 8 seconds. While it is possible to cure the resin in asshort a period as 3 seconds, it is imperative that completepolymerization throughout the ink film thickness is achieved. In somesituations it maybe desirable to irradiate the circuit board from boththe top and bottom thereof due to the actual protrusion of the spheresthrough the ink film thickness. This not only ensures completepolymerization but it also causes some of the resin to shrink away fromthe highest point of the spheres, thus enhancing the solderability ofthe trace.

In a preferred embodiment of this second system a samarium cobalt magnet18 is employed which has the following properties:

    ______________________________________                                        Peak energy density                                                                             (BdHd)    max ×10-6 24.0                              Residual Induction                                                                              Gauss     10200                                             Coercive force    Oersteds  9200                                              Saturation Magnetizing Force                                                                    Oersteds  >20,000                                           ______________________________________                                    

Where larger circuits are to be exposed to magnetic levitation it ispreferred to use a samarian cobalt magnet due to its greater peak energydensity which is in the range of 20.0 to 28.0. It is important that whenthe magnet face is ground with abrasive wheels or the like that liberalamounts of coolant are employed to minimize heat cracking and chippingwhich have an adverse effect on the uniformity of the magnetic field towhich the circuit trace is exposed.

Another embodiment of the magnetic device employed in the presentinvention is shown in FIGS. 7 and 8.

The magnetic field to be used for levitating the spheres must not be auniform field. This is the reason for the polepiece 22 being addedversus a curved magnet 18 previously described. In this alternativedevice, there must be a strong gradient of the field, in order that theupward force on one of the induced poles exceed the downward force onthe other, otherwise there will be no tendency for the spheres to risein the ink film thickness.

Using this alternate device, it is much easier to homogenize the flatmagnet in order to achieve uniform peak field density and use the yokedesign with a cold roll steel polepiece to create a certain area ofnon-uniformity.

This alternate design provides the necessary field gradient since thelevitating field is constructed in a configuration that provides auniform value of the gradient of the vertical magnetic field along onehorizontal direction and passes the material to be levitatedhorizontally through it in a direction perpendicular to the first. Thus,the necessary gradient is established by supporting the rare-earthmagnet above the path of the circuit while providing a steel polepieceunderneath that path to guide the field lines into the return part ofthe magnetic field. This will subject the spheres to a lifting force asthey enter and again as they leave the region of the magnetic field.

A vertical gradient in the magnetic field necessarily is accompanied bya horizontal gradient, however this alternate device minimizes thiscondition by configuring the field such that the horizontal gradient isin the direction of motion of the substrate. There will be somehorizontal displacement of the spheres, but, being first forward andthen backward along the line of travel, it does not adversely affect thefinal resolution.

The maximum remanent magnetism of the magnetite spheres is 1.5 emu/gm.This compares with the minimum value of the saturation magnetization of81 emu/gm. This means that there will be very little residualmagnetization in the cured conductive trace and will have no effect onthe ability of the cured trace to conduct an electric current.

In both the first and second systems described above adhesion of theprinted circuit trace or pattern is governed by the U.V. curable inkcomposition and substrate interactions.

Adequate wetting by close contact of the U.V. curable ink compositionwith the substrate is essential for the attainment of satisfactoryadhesion. Substrate/ink composition interactions can be both chemicaland physical in nature. Either ions or covalent bonding between the inkcomposition and substrate can evolve powerful adhesion forces. However,owing to the transient time gap and relatively low temperature, covalentbonding forces are not easy to create for photopolymerizable systems.

Generally speaking, adhesion is not a problem for the curing on poroussubstrates such as paper and plastics.

Compared to free-radical polymerization, cationic photopolymerizationdoes show some advantages on adhesion:

(i) Ionic bonds between the ink composition and the substrate are morelikely formed.

(ii) After curing, the saturated epoxide system will have less shrinkagethan the polymer cured from unsaturated compounds. Sometimes theseshrinkage stresses in a high-density crosslinked ink composition arestrong enough to tear the coating off the substrate.

Ink compositions for non-porous substrates such steel and copper willrequire the addition of a wetting agent.

In case adhesion is a problem, post-bake may be required to anneal thefilm while cooling, thereby relaxing the residual cure stress.

The present invention also relates to a composition for use in shieldingenclosures housing electronic equipment and to the enclosure providedwith this shielding composition.

Prior to 1975 most enclosures for business machines were constructed ofmetallic materials, such as die cast zinc or aluminum or sheet metalaluminum or steel. In the mid-1970's plastics processors developed as apotential challenger to the metal enclosures, a structural foam moldedcabinet using some of the newer engineering resins. The structural foamprocess was developed by Union Carbide in the 1960's and by themid-1970's the technology had been developed to the point where foamedunits could offer comparable strength and lighter weight to cold rolledsteel. By the early 1980's the continued miniaturization of electroniccomponents reduced many of the enclosures to a size where straightinjection molded units could be used instead of metal or structuralfoam.

Further, the conversion from metal to plastic enclosures is of directconcern to the EPA because it is certain to increase the incidence ofemissions of volatile organic compounds (VOC's) into the environmentfrom the surface coatings of these plastic parts. Metal enclosures arecoated either to improve their appearance or to protect them fromenvironmental stresses. The coatings normally used on metal cabinets andcases are not high in solids or VOC's. Plastic parts, on the other hand,are coated for three major reasons:

a) to improve their appearance

b) to protect the plastic part from physical and chemical stress and

c) to attenuate EMI/RFI.

Because of the nature of plastic and the different requirements ofconventional coatings to ensure long-term adhesion to the plasticsubstrate, these surface coatings are high in solids in an organicsolvent formulation. It can be assumed that the level of VOC emissionswill increase as the conversion from metal to plastic proceeds and aselectronic equipment manufacturers specify an ever growing amount ofsurface coatings for decorative and more importantly EMI/RFI attenuationpurposes.

The major problem observed by the shift from metal enclosures is theabsence of inherent EMI/RFI and ESD capabilities. Where cold rolledsteel or die cast zinc was used for an enclosure these metals providedthe conductivity to deal with EMI entering or exiting the enclosure.Only the apertures where emissions could leak in or out (e.g., seams,air vents, cable entry points etc.) had to be shielded for EMI. In thecase of nonconductive plastics, every square inch of surface as well asthe apertures, have to be shielded for EMI, as well as treated for ESD.

To overcome the disadvantages associated with the use of a plasticenclosure for electronic equipment the inventor has discovered that thecomposition of the present invention, i.e. a U.V. curable compositioncomprising a suspension of silver coated magnetic particles in acycloaliphatic epoxy resin binder and a cationic photoinitiator providesan effective shield against EMI (electromagnetic interference), RFI(radio frequency interference) or EDS (electrostatic discharge).

While the term "EMI shielding" may be somewhat loosely employed in theplastics and electronics industry, nonetheless, the following is anexplanation of what this term means in this field.

There is both natural and man-made electromagnetic radiation (EMR), andany man-made piece of digital electronic equipment with a clock emits inoperation an amount of electromagnetic energy; the faster the clocksetting, the greater the amount of energy emitted. Digital devicesutilizing small integrated circuits and microprocessors can generate asignificant amount of EMR. This EMR can travel out to come into contactwith the conductors of another digital device such as the power cable,printed circuit boards, and various connecting wires within the device.The EMR generates a current independent of the operating current byinducing current flow in the conductors. A circuit board will receiveand respond to this current just as if it were receiving the regularoperating current. In other words, this random signal is givingextraneous electrical "instructions" to the device that can causeunwanted program or data changes. In this example EMR becomes EMI.

10 KHz is the lower boundary for regulations enforced by the FCC. Theproblem of emission of and susceptibility to EMR in this frequency rangeis referred to as radio frequency interference (RFI). This is the rangethat U.S. and international regulatory agencies such as the FCC and VDEare concerned with and is the range that the composition of the presentinvention has been found to control.

Static is a natural phenomenon where a rapid flow of electrons movesfrom an electrically charged object to another object to equalize thepotential difference between them. The rapid flow of electrons can alsoinduce current flow in conductors by creating EMR. For example, underlow humidity conditions a person walking across a room can act as acapacitor and build up a charge potential of 10,000 volts or more. Thisphenomenon is known as electrostatic discharge (ESD).

Any piece of electrical or electronic equipment when put into operationgenerates electromagnetic waves composed of electrical (E vector) andmagnetic (H vector) impulses. The magnetic impulses can penetrate allplastic and metal materials except ferrous materials. But magneticsignals terminate fairly rapidly over a short distance. Electric fields,on the other hand, can penetrate plastics, but not grounded metals.These signals can travel much greater distances. Thus, inelectromagnetic energy the electrical field is the more potent andpotentially more disruptive force.

Computing devices generate timing signals and pulses at rates of overone million pulses per second in order to carry out control and logicfunctions quickly and efficiently. This electromagnetic and radiofrequency energy is radiated into space and conducted as well throughmedia such as power lines. This energy has the potential to interferewith all forms of conventional electrical and electronic-radio,telephone and television reception.

The most common form of ESD can generate EMI, such as walking across acarpet and touching an object like an electronic device. Theconductivity required of the receptor to protect against ESD is not asgreat as that required against EMI from a computing device. So that evena plastic enclosure which is inherently insulative can protect againstsome mild forms of ESD. But nevertheless, charges can build up inplastic and they will remain there until sufficient charge isaccumulated to be discharged to a grounded susceptor. If the unit is notproperly grounded, a sufficient charge may build up and the cumulativeelectron flow can arc to the circuitry and cause an EMI problem.

To achieve electromagnetic compatibility (not generating nor beingsusceptible to EMI, RFI, or ESD) an electronic unit has to be designedin such a way to minimize these emissions or susceptibility. Theenclosure also has to be conducted in such a way as to trap any residualinward or outward EMI emissions.

The control of electromagnetic interference involves essentially ashield to contain or envelop the signals. When an electromagnetic waveencounters a shield, it will be reflected back to some extent if theimpedance of the wave and that of the shield differ significantly.Highly conductive metals have a low impedance and they serve to reflectback the electromagnetic wave. In contrast, a low impedance magneticwave encountering metal with a close match in impedance will result in atransfer of energy through the metal. Magnetic waves can be verydifficult to shield. Over greater distances the electric field componentdominates, and it is this factor which has to be dealt with throughEMI/RFI shielding.

Electromagnetic waves pass through space or through nonconductive solidmaterials at 3×10⁸ m/sec. When shielding on a nonconductive material,such as most plastics, is employed, these waves strike the shield andsome of their energy is reflected back just as light reflects off amirror. The rest of the energy may be absorbed in the shield, furtherattenuating the wave's strength. Technically, there may be a furtherloss of the field's strength when the residual energy reaches theexternal perimeter of the shielding.

It is an important point to note that in the context of comparing thevarrous shielding methods to the shielding composition of the presentinvention, the relative thickness of the shielding material has littleeffect on the reflected element of the wave, but it has a strong effecton its absorption. At higher frequencies reflection decreases andabsorption increases. So the greater attenuation of thicker barriers isimportant when dealing with higher frequency outputs.

It is also important to note at this stage that the absorption effect ofan EMI shield will differ depending on whether the shielding material isentirely or partly conductive. This is an important distinctionconcerning the comparison of the shielding effectiveness of solid metalsurfaces (ex: zinc arc spray) and metal-filled coatings (ex: nickelbased acrylic binders) and the composition of the present invention.

In summary, electromagnetic energy is an energy field radiating from anelectrical or electronic source containing both electric and magneticfield components. The field surrounding a highly electrically chargedobject is an electric field. Its presence is manifested by oppositecharge objects clinging together or like charged objects separating. Thefield surrounding a highly magnetically charged object is a magneticfield. Its presence is manifested by the attraction of other magneticmaterials in the same way that iron filings cling to a magnet.

In each case the electric and the magnetic fields are static; theirintensity is constant and there is no change in either their strength ortheir position. However, this can cause arcing. This arcing orelectrostatic discharge (ESC) is a serious problem in switchingelectrical circuits since the separation of the switch or circuitbreaker establishes an arc which must be extinguished in order to breakthe circuit. Static discharge can also cause problems through temporarydelays in the transmission of signals.

While metal cabinets or enclosures provide effective protection againstthe build-up of static due to their natural conductivity, plastics willnot. Effective dissipation of ESD is dependent on the placement of theelectronic components within the cabinet and other designconsiderations. However, in the area of EMI shielding which ispotentially far more serious, the solutions have tended to center aroundproviding a metallic coating or infrastructure to the nonconductiveplastic to reduce the ESD to a minimum.

The static nature of ESD leads to relatively straightforward solutionsto the problem; the level of shielding required to eliminate ESD isgenerally fairly low. Electromagnetic fields, on the other hand, are notstatic; their intensity varies and their polarity alternates. The rateat which the field alternates is the frequency, measured in cycles persecond or Hertz; this applies to both the electric and the magneticfield components.

The effectiveness of an EMI shield, as measured by dB, is referred to asattenuation. Each 10 dB increment of attenuation or dissipation ofenergy provides a 10 fold improvement in shielding effectiveness. A 10dB attenuation results in a 90% attenuation of the force field; 20 dByields a 99% reduction; 30 dB yields a 99.9% reduction and so on. In thearea of computing devices generally EMI shielding in the 1-1000 MHzrange with an attenuation of 30-40 dB is sufficient to comply withgovernment regulations in 95% of all existing business machineapplications.

For electric fields reflection is very large relative to absorption andit occurs primarily on the surface of the part. This is why thinshields, such as those obtained by electroless plating, vacuumdeposition, and other thin film metal deposition technologies are veryeffective in attenuating electric fields. Conversely, the primaryreflection of magnetic fields is re-reflection within the shield. Thus,the attenuation of magnetic fields is best accomplished through thickskin shields, such as those produce by conductive coatings and zinc arcspray, and by high magnetic permeability of the shield material. Animportant point is that prior to the development of the shieldingmaterial of the present invention there was no one "perfect" shieldingmaterial. Unique to the shielding material of the present invention isthe high conductivity of the silver coating and the high magneticpermeability of the magnetite spheres.

The proliferation of EMI and the response to this growth by variousregulatory agencies is attributable to three basic forces. First, it isdue to the tremendous growth of electronics as an enhancement toproductivity in every aspect of modern life. Secondly, the typical pieceof electronic equipment is more powerful--in other words, the clocks areworking at a faster pace to input and output the data. Thirdly, therehas been a very significant conversion from metal to plastic enclosuresfor this equipment. And, whereas most metals offer a high level ofinherent conductivity, plastics are insulative in nature, so they aretransparent to EMI.

While there has been significant activity in the development ofinherently conductive plastics, nonetheless, those developed thus farwhile exhibiting modest amounts of conductivity, suffer from, interalia,the following disadvantages:

a) the materials are only available in sheet form and

b) they are highly sensitive to moisture and air.

Hence, at this time, one principal way to render a plastic materialconductive is to metallize it--either by the incorporation of metalfillers into a plastic compound or by surface treatment using pure metalor metal based coating. While plastics may have replaced metals in thestructure of the enclosures, they still have to rely on the metals in aplastic/metal marriage to provide all the properties required of anenclosure that will comply with EMI/RFI regulations.

As can be seen from the data in the table below, the shieldingcomposition of the present invention exhibits significant advantages asan RFI and EMI absorber. Metal Relative Conductivities RelativePermeability

    ______________________________________                                        Metal    Relative Conductivity.sup.a                                                                   Relative Permeability.sup.a                          ______________________________________                                        Silver   1.05               1                                                 Iron     0.17            1,000                                                ______________________________________                                         .sup.a Relative to copper                                                

Further, the shielding material of the present invention contains novolatile organic compounds (VOCs), and it has been demonstrated that atwo mil coating of the shielding composition of the present inventionprovides the same shielding effectiveness as a three mil coating of zincarc spray or three mils of nickel acrylic coating. This range is in theneighborhood of 40-60 Db.

What is claimed is:
 1. A printed circuit board comprising anon-conductive substrate and at least one conductive line printed onsaid substrate, said line comprising a U.V. cured ink comprising acycloaliphatic epoxy resin containing 25-67 volume percent ofsilver-coated magnetite particles, said line having been subjected to amagnetic field sufficient to move the magnetite particles to a positionat or near the upper level of said line so as to increase theconductivity of said line to the desired level and cured by exposure toU.V. radiation in the presence of a cationic photoinitiator.
 2. Theprinted circuit board of claim 1 wherein said U.V. curable ink is curedalso in the presence of a plasticizing amount of a polyether polyol. 3.The printed circuit board of claim 1 wherein said non-conductivesubstrate is a polyolefin, a polycarbonate, a polyvinyl halide or apolyester.